1. Home
  2. >
  3. Hong Kong's Wild Stars

The Hermaphroditic Sea Slug

Originally published in Green Country, Issue 126 (Jun 2017)
Author: Green Power

Hermaphroditism is a feature of sea slugs. Whether an animal is “male” or “female” depends on the role it plays during mating: the semen depositor is the male, while the semen receiver is the female. However, among sea slugs, there may be more than two individuals mating at the same time. On such occasions, the sex roles keep changing. The same sea slug may be the semen depositor and receiver at the same time. It is indeed hard to classify the gender of the creature!

All sea slugs possess reproductive organs of both sexes. The male organ is at the right bottom side of the head, while the female organ is at the back. When two sea slugs mate, the male semen depositor will press its head against the back of the female semen receiver. However, if there a third party joins in the mating process, the roles may change. The newcomer will press against the back of the “male” sea slug, which then becomes a “female”, performing the role of both semen depositor and receiver. The situation changes again if yet another sea slug joins in.

It may sound weird to have concurrent mating of three, four or even five sea slugs. But collective mating, or a mating chain as it is called, is common in the sea slug world. This is to prevent competition and fighting among the same sex, and increase the chance of reproduction in the population. Sea slugs may mate for a few hours and even days!

“Noodles” spilling all over

Sea slugs belongs to the subclass Opisthobranchia. Several species of sea slugs can be found in Hong Kong waters, notably the brightly-coloured nudibranchs and sea hares, each of which has a pair of long rhinophores that resemble rabbit ears. In spring or summer, we may encounter piles of noodle-like stuff on the beach. These are the egg masses of sea slugs.

Sea slugs are very productive. A section of the egg mass can give rise to a few dozen offspring. Up to a million offspring can emerge from a complete mass of eggs. Yet, being at the lower level of the food web, sea slugs and their egg masses are food for many marine creatures. Only a fortunate few can reach maturity.

Pheromones are released by egg mass to transmit information. Researchers believe that the chemical information helps to enhance the mating of sea slugs and increases the chance of reproduction.


Though in the Mollusca, sea slugs are vulnerable to predators without the protection of hard shells. What is their strategy when threatened? The answer lies in the defence mechanism involving a venomous purple secretion that can paralyse the nerves of potential predators. Even when the venom is not effective against a predator, the purple secretion acts as a smokescreen that allows the sea slug to escape.

Photo from Pixabay
Sea slugs are commonly found in shallow waters and intertidal zones in Hong Kong. (On the top is Hypselodoris festiva and the down is Chromodoris lineolata
© Dr. Apple Chui
Sea slugs preparing for mating. The “male” semen depositor presses its head against the back of the semen receiver.
Purple venom is secreted by sea slugs, to scare away potential predators.
Piles of noodle-like sea slug egg masses can be found on beaches.
© Peggy Chung

Subscribe to Green Power's latest news

Contact Us

3961 0200 (T)
2314 2661 (F)
Rm. 2314, 23/F.,Gala Place,
56 Dundas Street, Mongkok, Kln.
Opening Hours:
Monday to Friday 9:30 a.m.-1:00 p.m.;
2:00 p.m.-6:00 p.m. (Closed on Saturday, Sunday and public holidays)